As is generally well known in the railway industry, truck mounted braking systems comprise a series of force transmitting members, levers and linkages which function to move a group of brake shoes against the wheels of a railway vehicle to effect stoppage of such railway vehicle. A pneumatically activated brake cylinder is typically provided in the braking system to initiate movement of this series of force transmitting members, levers and linkages to apply the brakes of the railway vehicle mounted to a truck assembly of said railway vehicle.
A well known type of truck mounted braking system is a TMX® truck mounted braking system (TMX® is a registered trademark to Westinghouse Airbrake Company, the assignee of the present invention). A currently used pneumatically activated brake cylinder for truck mounted braking systems generally comprises of an air cylinder piston which moves in a forwardly direction within a cylindrical member upon the application of pneumatic pressure thereto. A seal and/or diaphragm is provided on or adjacent a first end of the piston. This seal and/or diaphragm contacts the inner surface of the cylindrical member so as to provide an airtight chamber at one end of the cylindrical member such that application of pneumatic pressure therein and against the first end of the piston enables forward movement of the piston. A piston rod is attached at a second end of the piston and moves in response to the movement of the piston. An opposite end of the piston rod is connected to the end of a push rod which is, in turn, connected to a cylinder force transfer lever. This cylinder force transfer lever is connected through a series of force transmitting members and linkages so as to activate a braking sequence and apply the brake shoes to the vehicle wheels.
A disadvantage of this type of pneumatically activated brake cylinder is that due to regulations regarding the amount of air pressure which must be supplied into the brake cylinder, it is sometimes difficult to control the movement and/or force applied by the piston. Some countries require that a certain amount of pressure, such as at least 1-1.15 bar greater than atmosphere, be applied within the brake cylinder. During light load conditions, too much force applied by the piston can cause the brake shoe forces to be greater than necessary resulting in wheel skid.
Another disadvantage is that care must be taken in the maintenance of the seals and/or diaphragms within the cylindrical member to ensure that leaking of air does not occur, resulting in a loss of pressure and a reduced amount of force being applied by the piston/piston rod assembly. Also, when cracking and/or deterioration of the seals and/or diaphragms does occur, the air brake cylinder must be completely disassembled in order to repair or replace the defective components. The difficulty in determining the condition of the components lies in that the components are contained within the cylindrical member thus resulting in a need for disassembly for inspection purposes.
An additional disadvantage of the currently used air brake cylinders is their inability to accommodate piston bail or misalignment without leaking air. In addition, it is impractical to visually determine the proper relationship between the actual stroke of the cylinder and the brake shoe force during braking.
U.S. Pat. No. 6,116,385, Dual Force Range TMX Cylinder Using an Airspring Actuator teaches a pneumatically activated brake cylinder which comprises a cylindrical casing engaged with a railway vehicle braking system. A hollow piston assembly having a first surface and an opposed second surface is mounted for reciprocal movement within the cylindrical casing. There is at least one air spring actuator engageable with the first surface of the hollow piston assembly and an opposed inner surface of such cylindrical casing. An air communication means is in fluid communication with an interior portion of the at least one air spring actuator for allowing the application and removal of air from the air spring actuator during a brake application or a brake release, and a piston rod assembly is associated with the opposed second end of the hollow piston assembly. This piston rod assembly is capable of movement in an outward direction from the cylindrical casing upon actuation of the air spring actuator to initiate a braking sequence for the railway vehicle braking system. The air communication means comprises an air inlet means which is provided in the cylindrical casing and the air spring actuator to enable application of pneumatic pressure within the air spring to form a first air cavity.
A packing cup is provided on the hollow piston assembly producing a seal between the hollow piston assembly and the inner surface of the cylindrical member to form a second air cavity. An air inlet flange is also provided on the cylindrical member to enable the application into and the evacuation of air from the second cavity.
The teaching of U.S. Pat. No. 6,116,385, Dual Force Range TMX Cylinder Using an Airspring Actuator is incorporated herein by reference thereto.
Although the TMX® braking system offers improved performance of the airbrake cylinder in certain applications, there is a need for a simpler device having less components.